In electric power systems for transmitting electric energy, it is known to use stationary induction machines with windings comprising cables. “Electric power systems” here denotes systems for voltages exceeding 1 kilovolt and “stationary induction machines” here denotes non-rotating induction machines, i.e. transformers and reactors.
A problem with the known cable-wound induction machines, especially in applications where large currents occur, is the difficulty of efficiently diverting the excess heat generated during operation because of Joule-effect losses in the lead of the cable. “Excess heat” here denotes the heat that causes the temperature in the induction machine to exceed a predetermined temperature, which is higher than the ambient temperature. A known method of providing cooling is to create flow paths, in which a coolant is induced to flow, between the winding turns. Usually, the cooling is forced, i.e. the coolant is induced to flow with the aid of a pump or a fan device.
In the cooling arrangement known through WO 98/34239 A1, the winding is designed with spacing elements that separate predetermined adjoining winding turns from each other. Flow paths in which a fan device induces a gas to flow, usually air, are thus created in the winding. In this context, hoods are commonly used to guide the gas stream into the winding. However, the above-mentioned cooling arrangements exhibit a number of drawbacks. First, placing the flow paths between adjoining winding turns means that the winding occupies a relatively large volume. This makes the induction machine relatively large, which in certain applications can be disadvantageous, for instance in transformers where a high filling factor in the winding is desired. The hoods, which guide the air stream into the winding, also contribute significantly to the size of the induction machine and, moreover, make the induction machine expensive to manufacture. Secondly, the flow paths constitute impairments in the winding, as adjoining winding turns separated by a flow path do not support each other. These impairments can make the winding sensitive to the forces that arise during short circuits in the electric power system. Thirdly, the present trend of development is towards ever-higher currents in the induction machines, which requires an ever-higher flow velocity for the coolant in gas-cooled induction machines to provide sufficiently effective cooling. This entails a large consumption of energy in the fan device.
In another known cooling arrangement, flow paths are created in the form of cooling tubes of an electrically insulating material, usually a polymer material, which cooling tubes extend through the winding between the winding turns. A pumping device pumps a liquid, such as de-ionized water, through the tubes. However, such arrangements cooled by liquid exhibit the same drawbacks as the arrangements cooled by gas described above, as the flow paths increase the volume of the winding and reduce its capacity to withstand short-circuit forces. In addition, a further problem arises. The permeability to liquids, at least to a limited extent, of polymer materials poses a risk of the cooling liquid permeating through the cooling tube and into the insulating layer surrounding the lead in the cable. The cooling liquid, in combination with the electrical alternating field that arises around the lead when an alternating current runs through the same during operation, can form so-called water trees in the insulating layer. This is undesirable, as the formation of water trees weakens the electrical insulating strength of the insulating layer. The formation of water trees can also occur in the cooling tubes, which is not desirable either.
Another cooling arrangement is known through GB 2332557 A, which describes a power cable for high-voltage induction apparatus. The power cable comprises an inner support or cooling tube of metal, through which a coolant flows. The aim is to cool the power cable to cryostatic temperatures and the cooling tube in question consists of metal, for instance an alloy of copper and nickel.
A cable-wound induction machine with a cooling tube of conducting material wound with the cable displays a great disadvantage, however. The disadvantage is that the magnetic flux in the induction machine induces electric currents in the cooling tube. This results in the cooling tube being heated and undesired losses arising. This problem increases with the frequency and the rated output of the electric power system in which the induction machine operates.